Hey everyone - I have a 2003 Marshall JCM 2000 DSL 100 head. All original - even original tubes. The amp has never given me any problems and prior to March 2020, I used it with the band regularly. Recently, the amp has developed a low "hum" when I turn it on and it is on standby. It gets much louder when it take it off standby. The hum is the same whether there is a cable plugged into the input or not. No matter what guitar I use or what speaker cable or speaker cabinet I use, no change to the hum. To see if the problem was from lights, I unplugged all the lights and there in the dark the hum was the same. No change to the hum if I press any of the buttons on the amp or change channels. I have tried turning each knob to zero individually and that does not change the hum at all. Is there anything I can do or try before spending money to take the amp to a tech??? Thanks!